U.S. patent number 6,790,264 [Application Number 10/130,333] was granted by the patent office on 2004-09-14 for control of ammonia emission from ammonia laden fly ash in concrete.
This patent grant is currently assigned to ISG Resources, Inc.. Invention is credited to Rafic Y. Minkara.
United States Patent |
6,790,264 |
Minkara |
September 14, 2004 |
**Please see images for:
( Certificate of Correction ) ** |
Control of ammonia emission from ammonia laden fly ash in
concrete
Abstract
The present invention relates to a pozzolanic admixture
containing ammonia-laden fly ash, method for making the pozzolanic
admixture and method for controlling ammonia gas (NH.sub.3)
emission from cementitious slurries using the pozzolanic admixture.
The associated hypochlorite and ammonia reaction produces
monochloramine and chloride salts at relatively low concentration
levels harmless to concrete and concrete applications. The
resulting monochloramine and chloride salt products are stable and
do not dissipate into the air, thereby, eliminating odorous
emission that is produced from cementitious slurry containing
untreated ammonia laden fly ash. This invention relates to the
method of adding hypochlorites (OCl.sup.-) in the form of calcium
hypochlorite--Ca(OCl).sub.2, lithium hypochlorite--LiOCl, sodium
hypochlorite NaOCl or trichloro-s-triazinetrione--C.sub.3 N.sub.3
O.sub.3 Cl.sub.3 to the ammonia-ladden fly ash at dosage levels,
based on ammonia concentration in ash and stoichiometry, for a
complete or partial oxidation of ammonia to eliminate, or
respectively reduce, ammonia gas evolution from the high pH
cementitious slurries.
Inventors: |
Minkara; Rafic Y. (Kennesaw,
GA) |
Assignee: |
ISG Resources, Inc. (Salt Lake
City, UT)
|
Family
ID: |
22689771 |
Appl.
No.: |
10/130,333 |
Filed: |
September 23, 2002 |
PCT
Filed: |
March 07, 2001 |
PCT No.: |
PCT/US01/07207 |
PCT
Pub. No.: |
WO01/66486 |
PCT
Pub. Date: |
September 13, 2001 |
Current U.S.
Class: |
106/705; 423/237;
423/352; 436/9; 73/28.01; 73/23.2; 436/113; 423/238; 106/DIG.1 |
Current CPC
Class: |
G01N
33/383 (20130101); C04B 20/023 (20130101); A62D
3/38 (20130101); G01N 33/0054 (20130101); B01D
53/58 (20130101); C04B 18/08 (20130101); C04B
20/023 (20130101); C04B 18/08 (20130101); C04B
40/0028 (20130101); C04B 18/08 (20130101); C04B
20/023 (20130101); C04B 40/0032 (20130101); C04B
2103/0094 (20130101); Y10S 106/01 (20130101); Y10T
436/100833 (20150115); Y02W 30/91 (20150501); Y02A
50/25 (20180101); Y02A 50/20 (20180101); Y02A
50/2346 (20180101); A62D 2101/08 (20130101); Y02A
50/246 (20180101); A62D 2101/45 (20130101); Y10T
436/175383 (20150115); Y02W 30/92 (20150501) |
Current International
Class: |
A62D
3/00 (20060101); C04B 18/04 (20060101); C04B
18/08 (20060101); G01N 33/38 (20060101); G01N
33/00 (20060101); C04B 018/06 () |
Field of
Search: |
;106/DIG.1,705
;423/237,238,352 ;73/23.2,28.01 ;436/9,113 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3526756 |
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Jan 1987 |
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3711503 |
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Oct 1988 |
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DE |
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3732026 |
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Apr 1989 |
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DE |
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3802884 |
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Aug 1989 |
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DE |
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56166978 |
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Dec 1981 |
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JP |
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57019078 |
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Feb 1982 |
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JP |
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59010327 |
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Jan 1984 |
|
JP |
|
59029024 |
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Feb 1984 |
|
JP |
|
59059237 |
|
Apr 1984 |
|
JP |
|
Other References
K Ohlinger; T. Young; E. Schroeder; Wastewater Chemnistry- Struvite
Precipitation Kinetics; Scope Newsletter; Mar. 2000;
http://www.ceep-phosphates.org/scope/articles/scope36/scope36-06.htm;
Scope No. 36; Internet Web site..
|
Primary Examiner: Marcantoni; Paul
Attorney, Agent or Firm: Wegman, Hessler &
Vanderburg
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
Priority filing benefit of (1) International PCT application
PCT/US01/07207 filed Mar. 7, 2001, and published under PCT 21(2) in
the English language and (2) U.S. Provisional Application Serial
No. 60/187,628 filed Mar. 8, 2000.
Claims
What is claimed is:
1. A method for reducing ammonia gas evolution from cementitious
slurries when ammonia-laden fly ash is added to the slurry,
comprising the steps: a) blending an oxidizer with said
ammonia-laden fly ash to form a mixture; and b) mixing said
oxidizer and ammonia-laden fly ash mixture with said cementitious
slurry at a pH of about 12 to about 14 to thereby form stable
reaction products that do not dissipate into the air and to reduce
said ammonia gas evolution.
2. A method according to claim 1, wherein said oxidizer is a
hypochlorite containing oxidizer.
3. A method according to claim 2, wherein said hypochlorite
containing oxidizer comprises a member selected from the group
consisting of calcium hypochlorite, sodium hypochlorite, lithium
hypochlorite and trichloro-s-triazinetrione and mixture
thereof.
4. A method according to claim 3, wherein said oxidizer is blended
with said ammonia-laden fly ash in a molar amount of 0.25:1 to
about 3:1 based upon Cl:N.
5. A method according to claim 4, wherein said oxidizer is blended
in an amount of about 1:2 to about 2:1 of Cl:N.
6. A method according to claim 5, wherein said oxidizer is blended
in an amount of about 1.5:1 Cl:N.
7. A method according to claim 3, wherein said hypochlorite
containing oxidizer comprises calcium hypochlorite.
8. A method as recited in claim 1, wherein said mixing is conducted
at a temperature of about 50.degree. F. to about 90.degree. F.
9. A method for reducing ammonia gas evolution from cementitious
slurries when ammonia-laden fly ash is added to the slurry,
comprising the steps: a) determining the concentration of ammonia
in said ammonia-laden fly ash; b) blending a quantity of an
oxidizer with said ammonia-laden fly ash, wherein said quantity of
oxidizer is selected based on said concentration of ammonia; c)
then, mixing said oxidizer and ammonia-laden fly ash mixture with
said cementitious slurry at a pH of about 12 to about 14 wherein
said oxidizer oxidizes ammonia in said ammonia-laden fly ash upon
mixing said oxidizer and ammonia-laden fly ash mixture with said
slurry to thereby form stable products that do not dissipate into
the air; and d) depositing said mixed slurry for curing.
10. A method according to claim 9, wherein said concentration of
ammonia in said ammonia-laden fly ash is determined via a rapid
screening test procedure.
11. A rapid screening test procedure for determining the content of
ammonia in ammonia-laden fly ash, including the following steps: a)
collecting a representative dry sample of said ammonia-laden fly
ash according to ASTM C702; b) placing 100 ml of water in a clean
1000 ml flask; c) weighing out 25 g of said ammonia-laden fly ash
and adding said fly ash to said water while stirring on a stir
plate in said flask; d) closing said flask and allowing said water
and said fly ash to stir for 1 minute e) opening said flask and
adding 10 ml of 1 N sodium hydroxide and immediate stoppering the
flask; allowing said water, fly ash and said sodium hydroxide to
stir for 1 minute; testing the pH of said water and said fly ash;
and, if needed, adding more sodium hydroxide to bring the pH to 12;
f) preparing the ammonia detector rube for testing; g) inserting
said detector tube into a hand pump; h) inserting said detector
tube into a stopper hold, into said flask; i) extracting a gas
sample through said detector tube via said hand pump; and j)
reading the entire length of the discoloration immediately.
12. A method comprising mixing (a) ammonia laden fly ash, (b)
cement, (c) oxidizer, and (d) water to form a slurry, said oxidizer
oxidizing said ammonia upon mixing of said water to reduce ammonia
gas evolution from said slurry.
13. A method for reducing ammonia gas evolution from a cementitious
slurry when ammonia-laden fly ash is added to the slurry,
comprising the steps: a) blending a hypochlorite containing
oxidizer with said ammonia-laden fly ash to form a mixture; and b)
mixing said oxidizer and ammonia-laden fly ash mixture with said
cementitious slurry wherein said oxidizer oxidizes ammonia in said
ammonia-laden fly ash upon wetting said mixture while forming said
cementitious slurry to reduce said ammonia gas evolution.
14. A method according to claim 13, wherein said hypochlorite
containing oxidizer comprises a member selected from the group
consisting of calcium hypochlorite, sodium hypochlorite, lithium
hypochlorite and trichloro-s-triazinetrione and mixture
thereof.
15. A method according to claim 14, wherein said oxidizer is
blended with said ammonia-laden fly ash in a molar amount of about
1:2 to about 2:1 of Cl:N.
16. A method according to claim 13 further comprising determining
the concentration of ammonia in said ammonia-laden fly ash and
wherein the step of blending the oxidizer comprises blending a
quantity of the oxidizer with said ammonia-laden fly ash, wherein
said quantity of oxidizer is selected based on the concentration of
ammonia in the fly ash.
17. A method according to claim 13 wherein said oxidizer and
ammonia-laden fly ash mixture is mixed with said cementitious
slurry at a pH of about 12 to about 14.
18. A method for in-situ oxidation of ammonia in ammonia-laden fly
ash to reduce ammonia gas evolution when the ammonia-laden fly ash
is wetted, the method comprising blending a hypochlorite containing
oxidizer with said ammonia-laden fly ash to form a mixture and
mixing said oxidizer and ammonia-laden fly ash mixture with water
to form a slurry, wherein said oxidizer oxidizes ammonia in said
ammonia-laden fly ash upon wetting to reduce said ammonia gas
evolution.
Description
BACKGROUND OF THE INVENTION
Fly ash produced at coal fired power plants is commonly used in
ready-mixed concrete as a pozzolanic admixture and for partial
replacement for cement. Fly ash consists of alumino-silicate glass
that reacts under the high alkaline condition in cementitious
slurry to form additional cementitious compounds when the fly ash
is added to the cementitious slurry. Fly ash is an essential
component in high performance concrete. Fly ash contributes many
beneficial characteristics to cementitious compounds including
increased density, long term strength, decreased permeability,
improved durability against chemical attack, and improved
workability of freshly placed material.
Coal burning power stations commonly inject ammonia or ammonia
based reagents into associated flue gas containing fly ash in an
effort to: (1) enhance electrostatic precipitator (ESP) performance
to reduce opacity and (2) remove nitrous oxide (NO.sub.x) using
selective catalytic reduction (SCR) and selective non-catalytic
reduction (SNCR) technologies to meet NO.sub.x emission
regulations. Ammonia injection into the flue gas for ESP, SCR and
SNCR performance enhancement commonly results in the deposition of
ammonia on the fly ash. Also, gas phase reaction of SOx and
NH.sub.3 in the flue gas results in the deposition of ammonium
salts on the fly ash in the form of ammonium
sulfate--(NH.sub.4).sub.2 SO.sub.4 and ammonium bisulfate--NH.sub.4
HSO.sub.4. In both SCR and SNCR processes, NO.sub.x is reduced
using ammonia to produce nitrogen gas (N.sub.2) and water (H.sub.2
O) vapor according to the following reaction:
The degree of ammonia contamination in the fly ash, and associated
concentration levels, vary among power plants depending on the rate
of ammonia injection, the performance of SCR or SNCR process, the
amount of SO.sub.3 in the flue gas and the associated operating
conditions of the boiler and air pollution control devices. It has
been observed that fly ash produced from high sulfur eastern
bituminous coal (Class F fly ash) adsorbs more ammonia than fly ash
produced from low sulfur western sub-bituminous coal (Class C fly
ash). As previously mentioned, the presence of sulfur in the flue
gases increases the associated deposition of ammonia in the form of
(NH.sub.4).sub.2 SO.sub.4 and NH.sub.4 HSO.sub.4. The high alkaline
condition of Class C ash inhibits ammonia cation (NH.sub.4.sup.+)
formation. Typical ammonia concentrations on fly ash, as a result
of ammonia injection, ranges between 50-120 mg/kg for SCR generated
fly ash, 250-600 mg/kg for SNCR generated fly ash, and 700-1200
mg/kg for ESP generated fly ash.
When ammonia-laden fly ash is used in cementitious slurry
applications, the ammonium salts dissolve in water to form ammonia
cations (NH.sub.4.sup.+). Under the high pH (pH>12) condition
created by cementitious alkali, ammonium cations (NH.sub.4.sup.+)
are converted to dissolved ammonia gas (NH.sub.3). Ammonia gas
evolves from the fresh cementitious slurry into the air, exposing
workers. The rate of ammonia gas evolution depends on ammonia
concentration, mixing intensity, exposed surface, and ambient
temperature. Ammonia has no measurable effect on concrete quality
(strength, permeability, etc.). Ammonia gas odors could range from
mildly unpleasant to a potential health hazard. Ammonia odors are
detected by the human nose at 5 to 10 ppm levels. The OSHA
threshold and permissible limits are set at 25 and 35 ppm for the
time weighted average--eight-hour (TWA 8-hr) and the short term
exposure limit--fifteen-minute (STEL 15-min), respectively. Ammonia
gas concentration of 150-200 ppm can create a general discomfort.
At concentrations between 400 and 700 ppm ammonia gas can cause
pronounced irritation. At 500 ppm, and above, ammonia gas is
immediately dangerous to health; at 2,000 ppm, death can occur
within minutes.
Other than OSHA exposure limits, there are no regulatory, industry
or ASTM standards or guidelines for acceptable levels of ammonia in
fly ash. However, based on industry experience, fly ash with
ammonia concentration at less than 100 mg/kg does not appear to
produce a noticeable odor in ready-mix concrete. Depending on site
and weather conditions, fly ash with ammonia concentration ranging
between 100-200 mg/kg could result in unpleasant or unsafe concrete
placement and finishing work environment. Fly ash with ammonia
concentration exceeding 200 mg/kg produces unacceptable odor when
used in ready-mixed concrete applications.
In addition to the risk of human exposure to ammonia gas evolving
from cementitious slurry produced using ammonia-laden ash, the
disposal of the ammonia-laden fly ash in landfills and ponds at
coal burning power stations also creates potential risks to humans
and the environment. Ammonium salt compounds in fly ash are
extremely soluble. Upon contact with water, the ammonium salts
leach into the water and are carried to ground water and nearby
rivers and streams causing potential environmental damage such as
ground water contamination, fish kill and eutrophication. Ammonia
gas could also evolve upon wetting of alkaline fly ashes, such as
those generated from the combustion of western sub-bituminous coal.
Water conditioning and wet disposal of alkaline, ammonia-laden, fly
ash exposes power plant workers to ammonia gas.
SUMMARY OF THE INVENTION
The present invention relates to the addition of a chemical
oxidizing agent to dry fly ash containing concentrations of
ammonia. The chemical can be added and blended with the dry fly ash
at any point between the fly ash collection system at the power
plant and final delivery to the ready-mixed customer, or at the
point of use at the ready-mixed customer site. The pre-blended
chemical oxidizing agent does not react with ammonia in the dry fly
ash; the chemical oxidizing agent is released during the wet slurry
mixing process. Once the ammonia-laden fly ash is introduced in the
cementitious slurry, ammonium salts from the ammonia-laden fly ash
dissolve. The high alkaline (high pH) condition of the cementitious
slurry converts the ammonium cations (NH.sub.4.sup.+) to dissolved
ammonia gas (NH.sub.3). Without the chemical oxidizing agent,
ammonia gas (NH.sub.3) evolves from the cementitious slurry during
mixing, transportation, pouring and placement.
More specifically, this invention relates to pre-treated
ammonia-laden fly ash and to methods of treating the ammonia-laden
fly ash. Addition of the chemical oxidizing agent with the dry
ammonia-laden fly ash prior to incorporating the fly ash into
cementitious slurries results in chemical conversion, via
oxidation, of ammonia into harmless products. Thereby, the exposure
risk of the ammonia gases (NH.sub.3) is limited.
The preferred chemical treatment reagents are strong oxidizers such
as hypochlorites (OCl.sup.-) commonly found in the form of
Ca(OCl).sub.2, NaOCl, LiOCl, trichloro-s-triazinetrione (trichlor),
etc. and are added to the ammonia-laden fly ash. Preferably, the
oxidizers are added in dry form to the fly ash, but it is also
possible to spray a dilute solution (containing up to about 30%
oxidizer) onto the fly ash. At present, calcium hypochlorite is
preferred. The reagent is activated upon water addition and reacts
with dissolved ammonia in the ash or concrete slurry to form
primarily monochloramine (NH.sub.2 Cl). An overdose of the
hypochlorite reagent would further oxidize monochloramine to form
nitrogen gas (NH.sub.2) and chlorides.
As used herein, the phrase hypochlorite containing oxidizer is used
to denote compounds that include the hypochlorite moiety or form
such moiety upon addition of water. For example, the trichor
compound forms hypochlorous acid and cyanuric acid upon water
addition. At elevated pHs, the hypochlorous acid ionizes to the
hypochlorite ion.
The basic aqueous phase ammonia oxidation reaction using
hypochlorite is as follows:
The rate of ammonia oxidation by hypochlorite depends upon pH,
temperature, time, initial dosage and the presence of competing
reducing agents. The pH condition of this reaction in Portland
cement based concrete and mortar is governed by the presence of
alkali from the associated cement hydration. The expected
cementitious slurry pH is between 12 and 14. The temperature of
freshly mixed concrete tends to be slightly warmer than the ambient
temperature as a result of the heat of hydration. The optimum
concrete temperature is in the range of 10 to 15.degree. C. (50 to
60.degree. F.), or lower for massive concrete pours, to avoid
thermal cracking. Concrete temperature should not exceed 33.degree.
C. (90.degree. F.). Time of reaction is also governed by
conventional and standard concrete practices namely mixing,
handling and placing guidelines. Ready-mixed concrete batches are
mixed for at least 5 to 10 minutes. ASTM C94 requires the concrete
to be placed within 90 minutes of mixing. The ammonia, in
ammonia-laden fly ash and concrete mixtures, represents the most
readily available reducing agent to react with hypochlorite. The
chloramine forming reaction of ammonia and hypochlorite in water
are 99% complete within a few minutes. Theoretically, a 1:1 molar
ratio of hypochlorite to ammonia (Cl:N) is needed to produce
monochloramine. Further increases in the molar ratio of Cl:N result
in further oxidation and formation of nitrogen gas and chloride
salts.
Fly ash and Portland cement are produced under strong oxidizing
conditions in boilers and kilns. Therefore, the fly ash and
Portland cement tend to be void of any reducing agents during
production; ammonia is introduced into the fly ash in
post-combustion air pollution control devices. Concrete aggregates
(i.e., sand and gravel) should be free of deleterious organic
substances that could otherwise exert a demand on hypochlorite. The
presence of some organic admixtures in concrete (e.g., air
entraining and water reducing agents) do not present any measurable
demand on hypochlorite since ammonia is a strong reducing agent and
the desired product, monochloramine, is also a reducing agent.
Other advantages and benefits will be apparent to one skilled in
the art when reviewing the specification in combination with the
drawings as described herein.
BRIEF DESCRIPTION OF THE DRAWING
The invention is more readily understood and appreciated by
reference to the following FIGURE:
FIG. 1 is a graph, resulting from the rapid screening test
procedure in accordance with the present invention, showing
detector tube reading versus ammonia in fly ash.
DETAIL DESCRIPTION OF THE INVENTION
A molar dosage of Cl:N between 0.25:1 and 3:1, preferably between
1:1 and 2:1, and most preferably 1.5:1, is sufficient to reduce
ammonia and prevent ammonia gas evolution from cementitious
mixtures containing ammonium compounds. For example, using 1:1
molar ratio, the theoretical amount, in kilograms (kg.), of
Ca(OCl).sub.2 per ton of ash needed to oxidize 100 mg/kg, as N
ammonia to monochloramine, is 0.51. In the case of lithium
hypochlorite (LiOCl), and using 1:1 molar ratio, the theoretical
amount, in kg, of LiOCl per ton of ash needed to oxidize 100 mg/kg,
as N ammonia to monochloramine, is 0.42.
The composition in accordance with the present invention and the
methods of making and using the composition are now illustrated
with the aid of the following examples, which are included for
illustration purposes only and are not meant to limit the
invention.
EXAMPLE 1
The results of the effectiveness of oxidation using Ca(OCl).sub.2
at various Cl:N molar ratios are presented in TABLE 1; the results
are for fly ash (20%) and cement (80%) mixed with water in a 1
liter flask. The ammonia concentration in the fly ash used in this
experiment is 600 mg/kg. The ammonia gas (NH.sub.3) concentration
in the closed head-space of the flask was measured according to the
rapid test procedure described below. Samples of the headspace gas
were extracted through a Drager.RTM. detector tube, or equivalent
device, after 2 minutes of mixing.
TABLE 1 Cl:N Molar Ratio NH.sub.3 Gas Concentration 0:1 >70 ppm
0.25:1 65 ppm 0.50:1 55 ppm 0.75:1 50 ppm 1:1 30 ppm 1.5:1 7.5 ppm
2:1 2.5 ppm
EXAMPLE 2
The results of the effectiveness of oxidation using Ca(OCl).sub.2
at various Cl:N molar ratios are presented in TABLE 2; the results
are for mortar mixes consisting of 30% cementitious materials
(cement and fly ash at 20% replacement), 70% sand and 0.6 w/c
(water to cementitious materials) ratio. The ammonia concentration
in the fly ash used in the mortar mixes tested at 300 mg/kg. The
ammonia gas (NH.sub.3) evolution was measured immediately after
mixing (t=0), at 5 minutes after mixing and 10 minutes after mixing
using Drager.RTM. detector tubes, or equivalent. NT indicates no
test performed.
TABLE 2 Cl:N Molar Ratio NH.sub.3 at t = 0 min NH.sub.3 at t = 5
min NH.sub.3 at t = 10 min 0:1 50 ppm 50 ppm 50 ppm 0.5:1 NT 40 ppm
NT 1:1 40 ppm 30 ppm 20 ppm 1.5:1 20 ppm NT 5 ppm 2:1 20 ppm 2.5
ppm 2.5 ppm 3:1 NT 0 ppm 0 ppm
EXAMPLE 3
The results of the effectiveness of oxidation using Ca(OCl).sub.2
in concrete mixes containing fine and coarse aggregates at Cl:N
molar ratios of 0:1 (no treatment), 1.5:1 and 2:1 are presented in
TABLE 3; the results are for concrete mixes consisting of 13%
cementitious materials (cement and fly ash at 20% replacement), 38%
sand, 49% gravel and 0.6 w/c (water to cementitious materials)
ratio. The ammonia concentration in the fly ash used in the mixes
tested at 600 mg/kg. The ammonia gas (NH.sub.3) evolution was
measured immediately after mixing (t=0) and at 15 minutes after
mixing using Drager.RTM. detector tubes, or equivalent.
TABLE 3 Cl:N Molar Ratio NH.sub.3 at t = 0 min NH.sub.3 at t = 15
min 0:1 >70 ppm >70 ppm 1.5:1 10 ppm 5 ppm 2:1 2.5 ppm 0
ppm
A rapid screening test procedure was developed as an integral part
of the present invention to determine the concentration of ammonia
in the ammonia-laden fly ash. The rapid screening test procedure
requires obtaining a representative sample of fly ash. A
predetermined amount of fly ash is mixed with a known volume of
water in a closed beaker to dissolve the ammonium salts. The pH of
the fly ash and water slurry is raised using sodium hydroxide to
over 12.0 to convert ammonium cations (NH.sub.4.sup.+) to ammonia
gas (NH.sub.3). The ammonia gas concentration in the closed
headspace of the flask is measured using disposable ammonia gas
detector tubes. A sample of the headspace gas is extracted through
the detector tube using a handheld air sample extraction pump. The
ammonia gas concentration in the beaker headspace is determined by
the color change, usually yellow to blue, on the graduated detector
tube. The ammonia gas concentration measured by the detector tube
is directly related to the concentration of ammonia in the ash
placed in the beaker.
The ammonia gas concentration measured using the rapid screening
test procedure, in accordance with the present invention, has no
direct relation to the ammonia gas concentration detected in
ready-mixed concrete trucks or to the ammonia gas emitted from
freshly poured concrete; this procedure only gives an indication to
the amount of ammonia present in fly ash. Actual concentrations of
ammonia out-gassing from large volumes of freshly mixed and poured
concrete structures would depend on the ammonia concentration in
ash, the ash content of the concrete, the size of concrete
structure, the location of the pour (e.g. below grade vs. above
grade), and weather conditions such as ambient temperature and,
especially, wind speed. The rapid screening test procedure is most
useful to determine the potential of ammonia out-gassing and to
evaluate mitigation options such as treatment with an oxidizing
compound.
The materials needed to perform the rapid screening test procedure
include: stir plate, 1000 ml Erlenmeyer flask, stopper, stir bar,
de-ionized or distilled water, detector tubes for ammonia with
0-100 ppm measuring range such as Drager.RTM. tubes, or equal, hand
pump for the detector tubes such as Drager.RTM. Accuro bellows
pump, or equal, 1 N sodium hydroxide, pipette and graduated
cylinders.
The rapid screening test procedure is as follows: (1) collect a
representative dry sample of fly ash using a core sampler or siphon
tube according to ASTM C702; (2) place 100 ml of water in a clean
1000 ml Erlenmeyer flask; (3) weigh out 25 g of the fly ash
material and add to the water while stirring on a stir plate in the
Erlenmeyer flask; (4) close the flask and allow the slurry to stir
for 1 minute; (5) add 10 ml of 1 N sodium hydroxide using a pipette
and immediately stopper the flask, allow the slurry to stir for 1
minute; the pH of the solution should exceed 12; if needed, add
more sodium hydroxide to bring the pH to 12; (6) prepare the
ammonia detector tube for testing according to manufacturer
directions; (7) insert the tube tightly in the hand pump making
sure that the arrow on the tube points toward the pump; (8) insert
the other end of the tube into the stopper hole mid-way into the
flask making sure that it is not immersed in the liquid; (9)
extract a gas sample through the detector tube by operating the
hand pump according to manufacturer directions; and (10) read the
entire length of the discoloration immediately. The color change
should be from yellow to blue. The reading on the tube indicates
the ammonia gas concentration in the beaker headspace in ppm. A
chart, as depicted in FIG. 1, is used to estimate the ammonia
concentration in the fly ash (mg/kg) based on the ammonia detector
tube reading (ppm). The chart of FIG. 1 was developed using ash
samples with known ammonia concentrations. The rapid screening test
procedure and the FIG. 1 chart are used to screen the ash and
determine the level of chemical oxidizer treatment needed using
this invention.
The effects of oxidation treatment on concrete quality are
positive. Monochloramine is highly soluble, stable and does not
effectively dissipate in the air. Monochloramine does not
disassociate into ions. It is a safe and commonly produced compound
for drinking water disinfection.
Overdosing of hypochlorite would result in further oxidation of
monochloramine (NH.sub.2 Cl) to dichloramine (NHCl.sub.2) and then
to trichloramine or nitrogen trichloride (NCl.sub.3).
Theoretically, it should require 3 moles of OCl for the complete
conversion of NH.sub.3 to NCl.sub.3. Further overdosing of
hypochlorite (molar ratio larger than 3:1) to complete the
oxidation would produce nitrogen N.sub.2 and nitrous oxide NO.sub.2
gaseous products and soluble chlorides. In the case of the reagent
calcium hypochlorite Ca(OCl).sub.2, overdosing would ultimately
produce calcium chloride CaCl.sub.2. Calcium chloride is commonly
used, at much higher concentrations (2% by weight of cement), as a
concrete admixture to accelerate early strength gain. The amount of
soluble chlorides added to concrete in overdose circumstances is
correspondingly low. For example, using an unlikely overdose
equivalent to a 5:1 molar ratio to treat fly ash containing 300
mg/kg ammonia, the amount of Ca(OCl).sub.2 added would be 6.12 kg
per ton of fly ash. Assuming complete oxidation to nitrogen gas and
calcium chloride, the amount of calcium chloride introduced is
calculated at 4.75 kg of CaCl.sub.2 per ton of fly ash which is
less than 0.1% by weight of cementitious materials based on 20%
cement replacement. Based on this example, the amount of chlorides
introduced to a typical concrete mix is about 0.01% or 100 ppm.
The effects of oxidation treatment using Ca(OCl).sub.2 on concrete
compressive strength was evaluated using mortar mixes with fly ash
treated with 0, 2.4, 3.6, and 4.8 grams of Ca(OCl).sub.2 per ton of
ash. The compressive strength results are shown, in TABLE 4, in MPa
for 3, 7, 14 and 28 days.
TABLE 4 Cl:N Molar g Ca(OCl).sub.2 /kg Ratio ash 3-day 7-day 14-day
28-day 0:1 0 20.2 23.8 27.0 25.1 1:1 2.4 21.0 23.4 26.7 24.3 1.5:1
3.6 20.6 24.3 24.5 24.5 2:1 4.8 22.9 24.3 29.1 25.6
Cementitious compositions in accordance with the invention comprise
a cementitious component such as Portland cement, fly ash, and an
oxidizer for controlling ammonia gas evolution from the mix. The
cementitious component may be present in a broad range of about
1-99 wt %, with the fly ash present in an amount of about 1-99 wt
%. The oxidizer is present in a molar amount relative to the
ammonia content of the fly ash as is stated above.
While there is shown and described the present preferred examples
of the inventive compositions and methods, it is to be understood
that the invention is not limited thereto, but may be otherwise
variously practiced within the scope of the following claims.
* * * * *
References